In telecommunication systems, it is often desired to estimate channel quality of a channel for sending data between a base station and a user equipment. When the channel quality has been estimated, the sending of data may be adapted to the estimated channel quality of the channel. As an example, by adapting the sending of data to the channel quality may be possible increase the data rate.
In a known telecommunication system, such as a Wideband Code Division Multiple Access (WCDMA) system, the channel quality is estimated by the user equipment and reported by the user equipment to the base station as a channel quality indicator (CQI). The channel quality indicator is based on measurements of signal strength for the sending of data and on measurements of signal strength for interference imposed to the sending of data. The measurements may be a measurement of signal-to-interference-ratio (SIR).
The signal strength for the sending of data may be estimated by measuring signal strength of reference signals, also called pilot symbols. The pilot symbols are transmitted in a slot of the WCDMA system. A transmission time interval (TTI) includes at least one slot. In the WCDMA system, 10 pilot symbols may be transmitted on a primary common pilot channel (P-CPICH) for each slot. Another 10 pilot symbols may be transmitted if a secondary common pilot channel (S-CPICH) is used. These pilot symbols are specified by a standard specification for high speed downlink packet access (HSDPA).
In the WCDMA system, a plurality of downlink channels may be associated to a plurality of codes for code division multiplexing. In this fashion, each downlink channel is associated to a respective code of said plurality of codes. A code that is not used for code division multiplexing of data may be referred to as an unused code or idle code. Thus, the signal strength for interference may be estimated by measuring on so called unused codes.
In the user equipment, the estimation of the CQI is based on combining weights. The combining weights are used to reduce so called colored interference. The user equipment comprises a Maximum Likelihood receiver (ML receiver), which calculates the combining weights, w, according to:w=R−1h Where R and h are estimates of an impairments covariance matrix and a net channel response, respectively. The net channel response may be estimated by the signal strength of the reference signals. The receiver uses the impairments covariance matrix to reduce interference. For example, so called colored interference may be reduced by means of the impairments covariance matrix via the combining weights as mentioned above. The estimate h may be generated based on reference signals. Thus, by improving an estimate of the net channel response the calculated combining weights may become more accurate. As a consequence thereof, the reported CQI, based on the combining weights, may become more accurate as well.
For the ML receiver, the number of pilot symbols as given by the HSDPA specification may not be enough for computing reliable estimates of the signal strength for the sending of data. It has been noted that ML receivers and approximate ML receivers are sensitive to errors in the estimates.
In order to improve the reliability of the estimates, it is known to provide an additional S-CPICH such as to increase power available for pilot symbols. As a result, the accuracy of the estimates may be improved. However, a problem with this known approach is that the power for additional pilot symbols is determined by a fixed setting. Therefore, it may be cumbersome to change the power for additional pilot symbols.